1. The most common rule for forming plural nouns is A. dropping the last letter and adding - ies. B. changing the y to i and adding - es. C. adding - es to the word. D. adding - s to the word.

The correct option is D.

Most nouns are turned into plural forms by adding letter 's' to the nouns. Other nouns required adding 'es' or changing the 'y' to 'i; and then adding 'es', but these are small in number compare to those that require adding letter s. Examples of nouns that required addition of letter s in order to form the plural form are: boy, girl, pencil, pen, school, parent, etc.

Your answer would be d adding - s to the word. have a great day